Security systems are employed to provide a restricted or controlled entry to a particular place or security area. Detailed information concerning the identity of the user and the associated passcodes are keyed to time schedules during which the user will be permitted access to the controlled area. The schedules include day, hour, and minute data. Typically the data, if sufficiently large, will be stored in a centralized location which also synchronizes the card reader clocks, if used. However, for many facilities, such centralized control for access control systems is undesirable or unfeasible. Under such conditions, the access control systems and card reader must be self-contained and maintain long-term reliability and accuracy.
Time data including day, hour, and minute information is frequently generated by a separate integrated circuit, typically an MM58174 by National Semiconductor Corporation of Santa Clara, Calif. If the crystal is not precisely adjusted, typically by an adjustable trimmer capacitor, time errors will accumulate, which can only be corrected by loading a corrected time into the circuit. Such adjustment or correction requires additional test equipment or digital logic which makes field adjustment difficult or impossible.